The Physiology of the Ascent of Sap

Table XIV. — -Effect of Variation of Temperature of the Petiole ON Transpiration It will be seen that during the fall of temperature of the petiole the transpiration underwent a decrease, while during the rise the activity was enhanced, in spite of the fact that the transpiring lamina itself was maintained at an uniform temperature. One of the crucial tests of the rhythmic nature of a tissue is its reaction to stimulus : sub-minimal stimulus inducing an acceleration of activity, maximal stimulus retarding or inhibiting it (p. 14). This test was applied to the various parts of the leaf in the following experiments.

Stimulation of lamina. — The electrical stimAilus has the special advantage that it can be easily graduated from sub-minimal or from maximal intensity. Electrical connections were made at two diagonal points on the lamina of Nauclea, which was stimulated by feeble induction shocks lasting for a minute. The normal transpiring activity was 116 ; but after feeble stimulation it was increased to 150, the enhancement being 29 per cent. The normal activity was restored in the course of twenty minutes. The electrical stimulus was now gradually increased. This induced a diminution of activity from the normal 116 to 87, or by 25 per cent. Further increase of intensity of stimulus induced a continuous diminution of activity, culminating in an arrest. Similar effects were also obtained with the leaves of Thunhergia.

Stimulation of the midrib. — I next applied stimulus to the midrib of the leaf ; this induced results similar to those of the stimulation of the lamina. Under feeble stimulation the transpiring activity was enhanced from the normal 150 to 190, i.e., an enhancement of 27 per cent. Recovery took place after fifteen minutes ; strong stimulation now depressed the activity by 67 per cent. Stimulation of the petiole. — Electrical stimulus was applied to the petiole of another leaf of Naticlea. Feeble stimulus was found to enhance transpiration by 34 per cent. ; stimulus of stronger intensity induced a depression by 44 per cent.

The foregoing experiments show that the tissues of the transpiring leaf, both petiole and lamina, give the reactions characteristic of rhythmic tissue to electrical stimuli, sub- minimal or maximal. We are now in a position to trace a complete picture of the physiological mechanism of the ascent of sap throughout the length of the plant. The pulsatory activity is initiated in the root, effecting the absorption of water from the soil. Similar activity in the cortex pumps the water up through the stem from cell to cell, and also injects water into the vascular tissue of the xylem (p. 175), along which it is physically transferred. The water conveyed by physio- logical conduction and physical convection reaches the leaf, where it is distributed along the veins and their numerous ramifications, and is eventually excreted or transpired into the intercellular spaces, whence it finds exit to the atmosphere outside by evaporation.

Physiological continuity as regards the ascent of sap thus exists in the plant, and any distinction of a specific activity of the root, of the shoot, or of the leaf is not merely arbitrary but highly misleading. For the excretion at the upper end results from the additive activities in all the regions of the plant. Brief reference may here be made to other modes of excretion, by glands, by water-pores, and from wounded surfaces. No hard and fast line can, however, be drawn in these different manifestations. Excretions in general

result from the additive action of cells throughout the length of the plant. The possession of a specially active Fig. 28. The Pitcher of Nepenthes, and Transverse Section showing the Glandular Structure terminal layer may, however, render the excretion by an organ more or less independent of the rest. Such a specially active layer is found in the glands of the pitcher of Nepenthes (fig. 28). The pronounced rhyth- mic activity of the layer is shown by the multiple electric responses ex- hibited by it (fig. 29). Excretion can therefore take place in the pitcher even in a condition of partial drought.

Further, active ex- cretion ma}^ take place even in the absence of glandular organs, for we shall find that Palms, which possess no glandular organs, exhibit active excretion even Fig. 29. Multiple Response given by the Glandular Tissue of the Nepenthes in the absence of root-pressure. Excretion by the nectaries is no doubt helped by the osmotic withdrawal of liquid by the concentrated sugar-solution outside ; but the first excretion must have occurred without this adventitious aid. Similarly, evaporation from the leaves promotes the maintenance of the turgor-gradient in the plant, by which the uniformity of transpiration is secured.

The activity of transpiration can be accurately deter- mined by the Bubbling Method. Taking the evaporation from a given surface of water as 100, the transpiration from an equal leaf-surface of Nauclea is 45, of Thunhergia 20, and of Bryophyllum calycinum 9. Accurate determination can be made of the relative transpiration from the upper and the lower surfaces of the leaf by the Bubbling Method. In Nauclea the ratio is 1:3-5; iri Thunbergia it is i : 4.

On the lower surface of an average-sized leaf of Nauclea there are 17 millions of stomata ; at a temperature of 30° C. the rate of transpiration from an individual stoma is 0*000028 mgrm. per hour. Transpiration persists for a length of time even after the smearing of both the surfaces of the leaf with vaseline. It is thus an active process not essentially dependent on evaporation. Transpiration is appropriately modified under physio- logical variations. Application of feeble electrical stimulus to the lamina enhances transpiration, while strong stimulus retards or arrests it. The effects described are also produced when the midrib, or the petiole, is stimulated instead of the lamina. When the lamina is subjected to a rise of tempera- ture, its transpiring activity is enhanced : the same result is obtained on raising the temperature of the distant petiole,

and lowering of the temperature of the petiole induces a depression in the rate of transpiration. These experiments prove that transpiration is a physiological process carried on by rhythmic tissue forming part of a rhythmic system continuous through- out the plant for the absorption and distribution of water. The Micro -Transpirograph — Effect of diminution of turgor on transpira- tion— Effect of stimulus — Opposite effects of stimulation of upper and lower surfaces of leaf — Effect of high frequency Tesla-current — Effect of electric waves — Effect of statical electric induction — Effect of thermal rays — Effect of light — Effect of red and of blue light — Effect of carbonic acid — Effect of ether and of chloroform — Summary.

In the previous chapter we found that transpiration is a phenomenon of active excretion ; it was also shown that it undergoes responsive variations under changes of temperature and under the action of electric stimulus. We shall in the present chapter consider the response of the excreting leaf to various further tests of its "pulsatory activity. These are : (i) the effect of diminished internal pressure ; (2) the effect of diverse modes of stimulation ; (3) the effect of light ; and (4) the action of anaesthetics. In addition an account of the action of thermal rays, of electric waves, of high frequency Tesla-current, and of statical electrical induction will also be given.

Detailed explanation has already been given of the very reliable and sensitive Method of Bubbling for the deter- mination of induced variations of transpiration. As it cannot, however, be made to record the induced variation, it was necessary to devise a second method, which would be automatic and would inscribe a record of the effects induced. This has been secured by the Micro-Transpirograph, whose automatic records afford all the necessary information as regards the normal rate of transpiration and its induced variations.

In fig. 30 a reproduction is given of the photograph of a moderately sensitive apparatus. An U-tube filled with water has a float, F, on one side and the transpiring leaf on the other. The free water surface bears a certain thickness of oil to prevent evaporation, the cut end of the short stem bearing the leaves (or the petiole of a single leaf) being im- mersed in the water below the oil. The float is attached Fig. 30. The Micro-Transpirograph The transpiring leaf on one side of an U-tube, and a float, F, on the other. The descent of the float caused t)y transpiration from the leaves is recorded by a writing-lever on a smoked oscillating plate of glass. For obtaining balance, the vessel v is raised or lowered by rack and pinion r. s^, stop-cock by the manipulation of which the writer may be adjusted at any position on the recording plate.

to a recording lever which inscribes the record on a smoked glass plate kept oscillating by a clock-work. For continu- ous record to exhibit diurnal periodicity, the sensitiveness may be considerably reduced by making the diameter of the U-tube large and reducing the magnification of the recording lever. For researches on the effect of various external agents on the rate of transpiration, the sensitive- ness may be exalted to any extent desired (i) by selecting an U-tube with a narrow diameter, and (2) by increasing

the magnification produced by the -recording lever. For ordinary purposes a magnification of 50 times by a single lever is quite sufficient. But this magnification can be greatly increased, and it is thus possible to record the loss of a milligram of water from the transpiring leaf. The greatest difficulty encountered in practice is that of the sticking of the float against the side of the U-tube, arising from unequal capillary action at the opposite sides. This may be obviated by making the tube in which the float moves perfectly vertical and preventing rotation of the float. The first is secured by a levelling arrangement of the tube, not shown in the figure. The float is attached to one arm of the lever which moves in a vertical plane ; jewel-bearings reduce its friction to a minimum. The float itself is made of a hollow aluminium tube which is very accurately turned. These precautions remove all difficulties in the perfect working of the apparatus. Subsidence of the float, caused by loss of water by transpiration, gives a record on the smoked glass plate, kept oscillating at intervals of twenty or thirty seconds according to different requirements. The record is taken on a moving plate, and the slope of the curve gives an indication of the rate of transpiration. The effect of any physiological variation is seen in the change of the slope of the curve, or in the widening or shortening of the intervals between the successive dots, the former indicating the enhancement, and the latter the depression of the rate.

There is, however, a far more sensitive method available which enables us to detect not only the immediate but also the after-effect of the external agent. This is the Method of Balance, in which the level of the float at the beginning is maintained constant, the rate of loss by transpiration being exactly compensated by an equal rate of supply. Under these circumstances the record becomes horizontal. The balance is easily secured by the supply of water from the vessel v, the rate of which is roughly adjusted by the stop-cock, the finer adjustment being produced by a slight raising or lowering of the vessel by means of a rack

and pinion. The normal rate is shown in the up-curve in the first part of the record of transpiration given in fig. 31 ; after the estabhshnient of the balance, the record is seen to become horizontal. The leaves were next subjected to a saturated atmosphere by holding a hollow vessel coated with moist blotting-paper over them. The effect of the reduced transpiration is seen in the imme- diate upsetting of the balance downwards. After the removal of the cylinder the balance became re-established as seen in the record, which again became horizontal. Had the after-effect been one of enhancement, the balance would have been upset in the op- posite direction with an upward movement. The horizontality of the record thus de- notes recovery to the normal rate.

We have now two independent means, namely, the methods of Bubbling and of the Micro-Tran- spirograph, for investigating the effect of external agents on transpiration. In some of the following cases both these methods were employed, the results of which will be found to be in perfect agreement with each other. Most of the experiments described below were carried out with a single leaf, with the cut end of the petiole in water. Fig. 31. The Record of Transpiration, N without, and M with Balance, which is upset downwards by subjecting Leaf for a Short Time to Saturated Atmosphere

Note re-estabUshment of balance, shown by record becoming horizontal, on restora- tion to original atmospheric condition. This condition may be artificially induced by a plasmo- lytic agent. The Bubbling Method was employed. The experiment was made with a leaf of Nauclca ; the cut end of the petiole was placed in water, and the normal rate of transpiration wr.s determined. On substituting a 2 per cent, solution of glycerin for the water, the effect was found to be a depression of transpiration. The normal activity, 120, of the leaf was reduced to 85 in the course of fifteen minutes. Transpiration persisted at the lower rate during the whole time of the experiment, which lasted for two hours.

We shall next study the effect of various modes of stimulation on transpiration. These are, the action of electric stimulus, which could be easily varied from sub- minimal to maximal ; the effect of mechanical friction ; and the action of light. Electrical Stimulus. — The effect of this, as determined by the Bubbling Method, has already been described in the last chapter ; it was shown that while feeble stimulus induced an enhancement, strong stimulus gave rise to the opposite effect of retardation and arrest. These results are confirmed by the Method of the Balanced Transpirograph. In this a record was first taken after securing exact balance, as shown in the horizontal record. An electric shock of feeble mtensity was next applied to the lamina, which is seen to upset the balance in an upward direction, indicating an enhancement of activity (fig. 32, e). The restoration of normal activity is seen to have taken place after a certain interval of time, as indicated by the record becoming horizontal.

The effect of a strong stimulus applied at e is seen in the next record (fig. 32, f) ; the balance is upset downwards, which indicates a depression of transpiration. After a certain interval of time the record is seen to have become horizontal, indicating the restoration of normal rate, and then there was an up-movement of the curve followed by a per- manent horizontal record. The result is significant, showing that while stimulus depresses activity, its after-effect may be an enhancement of activity. This was also found to be the case in the record of Desmodium pulsation (see fig. 4) . The

(e) Depression induced by strong stimulus. Note the en- hancement as the after-effect, indicated by the subsequent up-curve. opposite effects of small and of large doses of chemical agents may be regarded as a phenomenon analogous to the above ; a small dose being equivalent to a feeble, and a large dose to an intense, stimulus. Mechanical Stimulus.— Electric stimulation acts diffusely on the lamina, and it is therefore impossible to apply it locally on the upper or on the under side of the leaf. Mechanical stimulation, however, labours under no such difficulty, and the upper or the lower side may thus be stimulated, one surface at a time. This brought out a very interesting difference in the two responses, as will be seen in the following experiments.

Responses to stimulation of the upper and the lower surfaces. — For this purpose different leaves of Thunhergia were taken and the surfaces stimulated by rubbing them with a brush. The experiments were can-ied out in the following order. The normal activity of transpiration was first determined, after which the upper surface was stimu- lated, causing an increase in the rate of transpiration. The rate was once more determined after an interval of fifteen minutes, by which time it had returned almost to the original value ; this I will designate as the second normal. The lower side was next stimulated, and a dimin- ution \\as the effect. It was invariably found that while the stimulation of the upper surface induced an enhance- ment of transpiration, that of the lower surface caused a diminution of the rate. The following are typical results obtained with three different specimens ;

Table X\'. — ^'ARIATIOx of Transpiration by Mechanical Stimulation OF Upper and Lower Surfaces of the Leaf [Thunhergia) Normal ..... Stimulation of upper surface Second normal .... Stimulation of lower surface Normal ..... Stimulation of upj)cr surface Second normal .... Stimulation (jf lower surface Effect of stimulation of upper surface enhanced transpiration by 40 per cent. Effect of stimulation of lower surface depressed transpiration by The important result obtained from the above experi- ments is that, under moderate stimulation, the upper and the lower sides of the leaf exhibit responses of opposite sign. The following considerations may possibly offer an explanation of this difference :

1. The transpiration of the lower surface is the more effective, the excretion from this surface being about four times greater than that from the upper. Now friction of the lower causes a direct, and of the upper, an indirect, stimulation of the more irritable and effective lower surface. It will be shown (Chapter XVIII) that direct and indirect stimulation often give rise to responses of opposite sign. 2. In a dorsi- ventral organ, the lower side is, generally speaking, more irritable than the upper. Thus in the pulvinus of Mimosa the lower side I find to be eighty times the more excitable. There is reason to beheve that in the lamina also the excitability is greater on the lower side. Moreover, it has already been demonstrated (p. 93) that while a sub-minimal stimulus induces an enhancement of transpiration, a maximal stimulus retards it. Now an identical stimulus which is sub-minimal for the less excitable upper side of the leaf may prove to be maximal for the more excitable lower side. Hence it is probable that the friction of the upper surface acted as a sub-minimal stimulus, enhancing transpiration, whilst the friction of the under surface acted as a maximal stimulus, retarding it.

If one terminal of a Tesla-coil be connected with a plate of metal, the latter becomes charged with oscillatory current several hundred thousand times per second. The space round the plate now becomes the field of alternating hues of electric force. When a leaf is placed in this field, its transpiring activity undergoes a definite variation, as will be seen in the following experiment. A leaf of Thunbergia was placed at a distance of 15 cm. from the plate of metal in

connection with the Tesla-coil. The normal activity was 29 ; this was depressed to 22 after exposure to the field of alternating current, the variation being — 24 per cent. On the stoppage of the action of the coil, the leaf recovered its normal rate after an interval of twenty minutes. A plate of metal was held at a distance of 15 cm. above the leaf and parallel to it. The plate was charged by a Wimshurst-machine, alternately with positive and negative electrification. In both these instances the rate of trans- piration was found to be increased. Thus in a leaf of Thunhergia the normal rate of 38 was enhanced to 54 under electric induction, the increase being 30 per cent. This effect takes place when the plate is held parallel to the leaf so that the lines of induction are perpendicular to the leaf. There is, however, no change in transpiration when the surfaces of the plate and the leaf are perpendicular to each other, that is to say, when the lines of induction are parallel to the surface of the leaf.

In electric culture, the high tension net-work no doubt exerts a statical induction and thus enhances the normal transpiration on which the supply of inorganic food-material to the plant depends. But if the high-tension current were alternating, then the transpiration would undergo a diminution ; and the result would be the differential effect of statical induction and of alternating field of electric force. This latter effect may be eliminated by the use of a valve by which an uni-directional current can be maintained.

I have shown elsewhere that Hertzian Waves of sufftcient intensity diminish the rate of growth. ^ They have a parallel effect in the depression of the rate of transpiration. Thus the normal rate of transpiration in a leaf of Thiinbergia was 76 ; the effect of Hertzian Waves of short length acting for a minute was to induce an immediate diminution to 68 ; this continued for the next five minutes, by which time it reached the value of 62, the diminution being 18 per cent. After this the leaf exhibited a gradual recovery, and in the course of twenty minutes it attained the normal rate of 76,

Thermal radiation was produced by means of an electric heating-iron, the heat-rays being allowed to impinge on the leaf. The normal activity of the experimental leaf of Nauclea was 63, and the immediate effect was an enhance- ment of the rate of transpiration to 80 ; this increase con- tinued for ten minutes after the cessation of radiation, the activity being enhanced to 109, or an increase of 73 per cent. The increase is due to the rise of temperature, which has a very pronounced effect in enhancing transpiration. The increase of transpiration under ordinary light is to some extent due to the presence of heat-rays.